Multichannel time modulated electrical pulse communication system



M. M. LEVY 2,489,302

6 Shasta-Sheet 1 NOI.

CHANNEL CHANNEL HULT I CHANNEL TIME MODULATED ELECTR I CAL PULSE COMMUNICATION SYS TE MA 5 T E R PUL Sf GENERATOR PASSIVE LAY NETWORK 2'51 LI E Nov. 29, 1949 Filed July 2. 1945 I I l 1 7 Can-i F m;

Invenwr CHANNEL CHANNEL N017.

Nov. 29, 1949 M. M. LEVY IULTICHANNEL TIME MQDULATED ELECTRICAL PULSE COMMUNICATION SYSTEM 6 Sheets-Sheet 2 Filed July 2. 1945 CH3 CH4 aEiLJ-s- 67/ UQK N hwenlor go :1 flan: L101 Nov. 29, 1949 M LEVY 2,489,302

M. MULTICHANNEL TIME MODULATED ELECTRICAL PULSE COMMUNICATION SYSTEM Filed July 2. 1945 6 Sheets-Skeet 3 FIG. 3.

8 Ir Width 9/ Modulation A Tin m/cmseconds' (f dfmquency F: 21! 1-5 Kc) Auo ey LEVY 2,489,302

M. M. IULTICHANNEL TIME MODULATED ELECTRICAL PULSE COMMUNICATION SYSTEM 6 Sheets-Sheet 4 Nmr. 29, 1949 Filed July 2, 1945 In ventor M nukmi ow-E LEVY y I A Altney Nov. 29, 1949 M. M. LEVY IULTICHANNEL TIME MODULATED ELECTRICAL PULSE COMMUNICATION SYSTEM 6 Sheets-Sheet 5 Filed July 2. 1945 F/Gsb.

F/G/Ob.

Inventor Nov. 29, 1949 M. M. LEVY 9,3 2

HULTICHANNEL TIME MODULATED ELECTRICAL PULSE COMMUNICATION SYSTEM Filed July 2 1945 6 Sheets-Sheet 6 F/G. /2a.

6, I 75/ I so 82 Inventor Patented Nov. 29, 1949 MULTICHANNEL TIME MODULATED ELEC- TRICAL PULSE COMIMUNICATION SYSTEM Maurice Moise Levy, London, England, assignor,

by mesne assignments, to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application July 2, 1945, Serial No- 602,804 In Great Britain May 26, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires May 26, 1964 13 Claims. (01.179-15) The present invention relates to multi-channel electrical pulse communication systems.

In systems of the type specified, each channel is allotted its respective portion of a cycle period during which the channel is made alive and the intelligence of the channel is transmitted as the time modulation of a pulse, for example the pulses of a channel are time phased between predetermined time limits, in accordance with the instantaneous amplitude of the signal wave of that channel. Arrangements for successively and cylically bringing the channels into use or making them alive are known as distributors, there being normally a distributor at the transmitter and a distributor at the receiver working in synchronism. The distributor functions as a channel selector.

In a system of the type specified it is desirable that the whole of a channel period should not be of any greater duration than the limits of modulation oi the time modulated pulses, and thus the channel period coincides with the channel modulation limits, which in general will be the same for all channels of a system. Furthermore, in order that the whole of the distributor cyclic period shall be occupied by channels substantially no time period is usually allowed between the termination of one channel period and the commencement of the next. In such a case an essential factor of the system is that no two adjacent channels should overlap in'time, otherwise cross talk or other interferences between the channels may result. It is the chief object of this invention to eliminate cross talk due to the overlapping in time of adjacent channels and to provide a multi-channel pulse communication system in which the period allotted to a channel does not commence until the preceding channel period has terminated.

This object is attained in accordance with the present invention by utilising pulses of rectangular wave form whose duration determines the duration of the portion of the distributor cyclic period constituting a channel period and oncetively adding to these pulses concurrently occurring pulses of another train of pulses having a smooth amplitude variation in phase with the rectangular pulses, one of the trains of pulses being derived from the distributor, and applying the addition resultant wave of a channel together with the intelligence wave of that channel to a pulse generator to produce time modulated pulses in accordance with the varying amplitude of said intelligence wave. The pulse generator may comprise electron discharge tubes, for example as in and decreases from a maximum to a minimum or vice versa during a channel period. Suitable wave forms are, for example, a saw-tooth wave form, a' triangular wave form or sinusoidal wave form. Preferably the wave form should be linear, since a non-linear wave form introduces distortion unless special precautions are taken. Either the rectangular wave form pulse or the auxiliary pulse is obtained from the distributor system. One way of obtaining the time modulated pulses is by allowing varying portions of the crest of the resulting combined wave form to pass to the circuits of the channels in accordance with the instantaneous amplitudes of the signal waves of the respective channels. The portion of the crest of the wave passed in the circuit may then be utilised in any desired manner to produce the desired type of time modulation pulses. For example, the crest portions may be greatly amplified and amplitude limited to produce duration modulated pulses, or the crest portions of the linear wave forms may be applied to a. difierentiating circuit to produce time-phased pulses, single or double push-pull pulses being obtained, depending upon whether a saw-tooth or triangular auxiliary wave form is originally employed.

In practice the apparatus used for passing the crest of the resulting combined wave form may be, for example, an electron discharge tube type of amplifier steadily biassed at the potential of the mid-amplitude of the auxiliary portion of the combined wave form, the bias or cut-off potential of the tube being varied in accordance with the amplitude of the signal wave to be transmitted. It will be observed that the amplitude of the sig nal wave may possibly be so negative at times as to neutralise the amplitude of the positive resulting combined wave, so that no portion of the crest of the combined wave is passed through the circuit and no pulse will be transmitted in the system corresponding to the particular amplitude of the signal wave resulting in this state of affairs. A similar state of affairs may exist, utilising a negative resulting combined wave and the positive signal maximum. Such a missing pulse may have serious repercussions at the transmitof constant duration herein called a "Safeguard pulse is superimposed on the maximum of the combined wave form applied to the modulator at the end of said combined waveform so that such short pulse will at least be transmitted.

Alternatively, a device may be used to limit the signal amplitude to within predetermined operational limits.

It will also be observed that due to the rectangular portion or vertical leading and trailing edges of the combined wave form, signal amplitudes above a particular value will not produce any further time modulation of the derived pulses and hence no interference with adjacent channels can take place on account of the modulation.

In one embodiment of the invention each channel pulse is produced by triggering a multi-vibrator type of circuit at suitable intervals oftime, and the triggering is effected by a combined wave of trapezoidal form produced by superimposing a saw-tooth wave form on a rectangular wave form. The triggering voltage of the multi-vibrator is arranged to be the mean of the trapezoidal wave form and is varied by application of the sig.. nal wave to the multi-vibrator so that the point of triggering on the trapezoidal wave form is advanced or retarded in accordance with the instantaneous positive or negative amplitude of the signal Wave.

In utilising a multi-vibrator circuit, if it is not triggered periodically, and in all pulse circuits, care must be taken to avoid production of any irregular bias voltages due to the irregular accumulation of charges or to ensure that no varying bias is produced by irregular charge at the end of each period. Such conditions might exist when, as hereinbefore explained, triggering fails because the signal amplitude is too great and neutralises the trapezoidal amplitude. Hence either a safeguard pulse is added to the maximum of the trapezoidal wave form or is applied on each multi-vibrator circuit or the signal amplitude is limited. A safeguard pulse may be conveniently obtained by passing the current from the electron discharge device used for producing the trapezoidal wave form through an inductance and adding the voltage produced across the inductance to the trapezoidal wave.

One convenient form of circuit for producing waves of trapezoidal form comprises an electron discharge device having in its anode-cathode circuit a series-connected resistance-capacity combination and a by-pass impedance such as a resistance of high value. Rectangular pulses are applied to the input circuit of the device through a very large resistance. By biassing the grid or cathode of the device, it will limit the pulse amplitude and a constant current will appear between anode and cathode for the pulse duration and the condenser will gradually charge up providing an increasing voltage across the resistance capacity combination. The shunt resistance provides a discharge path for the condenser during the intervals between pulses. If the resistance-capacity combination is connected between cathode and earth, positive trapezoidal pulses are obtained and if between anode and high tension supply, negative trapezoidal pulses are obtained. The rectangular pulses for application to the electron discharge device may be obtained from a passive delay network or artificial line used as a distributor in a multi-channel system.

It will be shown hereinafter that a trapezoidal wave form obtained by the use of a resistance capacity network introduces distorting components into the transmitted time modulated pulses on account of the fact that the increase of voltage across the network with time between the time limits of the wave form, is not linear but varies exponentially. According to another feature of the invention this distortion is neutralised at the receiver by utilizing a demodulator which produces duration modulated pulses under the control of the received time modulated pulses and involving a resistance capacity network whose time constant is substantially equal to the time constant of the resistance-capacity network at the transmitter. The above considerations apply also to resistance-inductance networks.

This feature however is applicable to any type of modulator involving the time constant of a resistance capacity network for the production of time modulated pulses. The invention, therefore, also provides an electrical communication system in which the intelligence wave is transmitted as a time modulation of a series of electrical pulses and having at the transmitter a modulator arrangement for producing time modulated pulses in accordance with the instantaneous amplitude of the intelligence wave and involving the time constant of an impedance network, e. g. resistance-capacity or resistance-inductance network in the modulator circuit thereby introducing an exponential voltage variation and consequential distorting components into the transmitted signal characterised in this that at the receiver the demodulator comprises an arrangement for producing under the control of the received pulses duration modulated pulses whose voltage varies with time in an exponential manner according to the same exponent as the variation at the transmitter. If a resistancecapacity network is employed at the transmitter in the modulator and at the receiver in the demodulator the time constants of the two networks accordingly are the same.

While in the foregoing description combined waves have been referred to, it will be understood that the trapezoidal or other form of combined waves may not in fact exist and the same effect may be obtained for example by applying the rectangular pulse and the auxiliary wave form individually to an amplifier in such manner that the effects due to the waves in the output of the amplifier are additive.

The invention will be more clearly understood from the following explanatory description and of some embodiments of the invention taken in conjunction with the accompanying drawings in which:

Fig. 1 shows the transmitter of a multi-channel system with the circuit arrangement of a modulator embodying the invention in detail.

Fig. 2 shows several curveswhich will be used in the description of the operation of Figure 1.

Figs. 3, 4 and 5 are graphs showing harmonic content in a distorted pulse and referred to in the description.

Fig. 6 shows the circuit of another form of trapezoidal wave form generator.

Fig. 7 is an explanatory diagram.

Figs. 8a, 10a and 101) are alternative networks for use in producing trapezoidal type of wave forms having exponential variation with time.

Fig. 8b shows the wave form produced by the network of Fig. 8a.

Fig. 9 is a circuit diagram of another modulator embodying the invention. a

Fig. 11 is a circuit diagram of a demodulator embodying the invention.

Fig. 12a is the equivalent network of the network in Fig. 11 producing the trapezoidal type of wave form having exponential variation with time. 1

Fig. 12b shows the duration modulated pulse with the exponential wave form produced by network shown in Figure 12a.

Fig. 13 shows an alternative circuit arrangement of demodulator for producing duration modulated pulses having exponential waveform.

Referring to the drawings, Figure 1 shows diagrammatically the transmitting end of a multichannel electrical pulse communication system in which the intelligence wave of each channel is transmitted as a time phase modulation of a series of electrical pulses, and utilising as a distributor a passive delay network or artificial line indicated generally at I. A master pulse generator indicated by the block 2 may be of any known type of generator of pulses of rectangular wave form as indicated at 3. These pulses of rectangular wave form are fed to the input terminals of the delay network or artificial line i, and as synchronising pulses through a one way device t to a transmission line 5 where they are combined with the trains from the channels and passed to a carrier frequency modulator for transmission through the desired medium.

The delay network comprises a plurality of series connected cells, all alike and each com prising inductances and capacities and may he resistances and so designed to delay a current passed therethrough by equal intervals or time. Channel selector pulses of rectangular wave form are obtained from respective tapping points equally spaced along the network l as indicated at ti, ll, 9, 9 and u. The output from the end of the delay network 1! is fed back through an arm plifler indicated by block it to the master pulse generator 2' so as to stabilise the-repetition fre-== quency of this latter and to ensure that only one pulse at any moment is present on the delay network and thus eliminate the possibility of two channels being operated together.

The duration of the rectangular pulse 3 is made equal to the channel period and hence equal to the time delay produced between any two successive points on the delay network I.

The equipments for all the channels are the same and are generally indicated by the blocks il- -M, while the equipment of channel 11 is represented in detail within the broken line rectangle l5. This equipment comprises a pulse generator US which is shown in the form of a multivibrator consisting of two valves ill, it interconnected in well known manner and designed to'be blocked by a suitable negative bias until it strikes by the application of the combined rectangular wave and auxiliary wave together with the intelligence wave applied to the control grid 19 of valve H. In the particular embodiment of the invention shown a pulse of trapezoidal wave form is employed as the equivalent of the combination of a rectangular pulse and a pulse of sawtooth waveform.

The means for producing the trapezoidal waveform comprises an electron discharge valve Zll to the control grid of which rectangular pulses as shown at 2! obtained from tapping point 11 of the distributor delay network are applied and has in its anode-cathode circuit a network comprising a ing edge t coincides with the limit TLZ.

. tained from the anode circuit.

capacity 22, in series with a resistance 23 and a shunt resistance 24 connected across capacity 22. A positive trapezoidal wave may be obtained from the cathode of valve 20. To obtain a negative potential trapezoidal wave, the network 22, 23 and 24 is connected in the anode circuit instead of the cathode circuit as shown. The trapezoidal waveform and the intelligence wave, obtained for example from a microphone 25 are applied addi tively, through condenser 26 and resistance 21 respectively to the control grid IQ of valve ll of the m'ultivibrator I6. It will be observed that resistances 2'! and 28 and the microphone 25 are in series in the grid circuit of valve IT.

A saturated inductance coil 29 is included in the anode circuit of valve 20 for the purpose of producing the safeguard pulse at the end of the trapezoidal waveform. This coil 29 acts as a differentiating circuit to the current pulse of rectangular waveform passing through the valve on account of the pulse 21. on the control grid. In the present instance a positive pulse, is required at the end of the rectangular pulse. As shown, utilising positive pulses of rectangular waveform the short pulse obtained at the end of the rectangular pulse is positive and is assumed to be applied through condenser .30 to a grid of valve ll for simplicity. A one way device Si is provided to eliminate the negative pulse, if necessary.

Pulses of trapezoidal waveform are thus applied to the control grid ill of valve ll and the control grid 32 of valve it is biassed by means or a potentiometer so that the multivibrator triggers when the voltage on control grid ill is that represented by the mid-point of the trapezoidal waveform when applied alone to ill.

Pulses as indicated at ti l whose duration depend up the time constants of the multivibrator circuit and which is arranged to have an on period shorter, equal or longer than the duration of a trapezoidal pulse. are obtained from across the resistance Ill-ii in the cathode circuit of valve it. These pulses are of positive sign, but if pulses of negative sign are required, they may be ob The trailing edges of these pulses may occur after the termination of a channel period and hence, a short sharp pulse marking the leading edge which is time phase modulated is obtained by difierentiating the longer pulses at by means of the capacity .th and resistance tl. The positive and negative pairs of pulses as represented at it are produced and the negative trailing pulse is eliminated by means of a diode til and the positive pulse is applied to the transmission conductor 5 viav a diode it to combine it with the pulses from the other channel equipments. The diode M serves to isolate the channel equipments from each other. As indicated in. the drawing the output of each channel equipment is fed to the line 5 through a diode.

The operation of the arrangement will be clearer from the curves shown in Figure 2.. Curve a shows the synchronising pulses S fed to line 5 and the mean positions in time of the channel pulses of the respective channels are indicated by the arrows CH9 CH6. The pulses may 00- cupy any position within the time limits represented by the dotted lines. For example the pulse CHl may occupy any position between the time limits represented by lines TLl and TLZ. Curve b shows the rectangular pulse Whose leading edge I coincides with the limit TM and whose trail- The duration of the rectangular pulse thus defines the channel period.

Curve shows a pulse of sawtooth waveform whose duration is equal to the duration of the rectangular pulse bl. This, however, is not necessary, the pulse cl may have a greater duration as shown in broken lines c.

Curve d shows the resultant pulse after adding pulses bl and cl. The broken line d shows the resultant pulse after adding pulses bl and c. It will be observed that the leading and trailing edges of the rectangular pulse still define the channel period.

Curve e shows the pulse d2, similar to dl, for channel 2. It will be understood that similar pulses occur at successive periods for the remaining channels.

Curve I shows the trapezoidal waveform produced by the network 22, 23, 24 in the cathode circuit of valve 20, Fig. 1. The rectangular pulses, for example bl, Fig. 2, obtained from the distributor delay network 1, Fig. l, are applied to the control grid of valve 20 through a resistance ll of very high value, and constant current will flow between anode and cathode for the pulse duration. During the flow of current the capacity 22 gradually charges up, providing an increasing voltage at the cathode of valve 20 as indicated by the line 42 in curve I, Fig. 2. The leading edge bi is produced by the current flow through resistance 23 due to the leading edge of the rectangular pulse. When the rectangular pulse terminates, there is a sudden cessation of current in the anode-cathode circuit of valve 20, giving rise to the trailing edge t of the pulse, but the capacity 22 still maintains a potential across the resistance 24 and as the capacity discharges through resistance 24, the potential gradually drops to zero as shown by the curved line 1n. Curve 9 shows a similar trapezoidal pulse T2 for channel 2.

As hereinbefore stated, the bias on the grid of valve I8 is adjusted so that the multivibrator will trigger when the voltage on control grid IQ of valve l'l attains a predetermined-value which is equal to the mean value of the trapezoidal pulse. This value is represented in curves d-g, Fig. 2, by the broken line TV. The intelligence wave is applied to the valve I'I additively in series with the pulse, so that efiectively the trapezoidal pulse is raised or lowered with respect to the voltage level TV. In other words, the voltage level TV moves with respect to the pulse between the limits TV! and TV2. The points where these lines TVI, TV, TV2 cut the pulse as shown in curvej represent the triggering moments of the multivibrator l6. It will be observed that even though the signal amplitude may move the pulse above or below the limiting values of TV2 and TVl respectively, the triggering times of the multivibrator cannot exceed the limits. This fact can be seen from curve a in which TVS and TVl are beyond the limits TV3 cutting the leading edge I and TV cutting the trailing edge if of the pulse. Thus the vertical leading and trailing edges of the pulses limit the modulation within the channel time limits.

Curve h, Fig. 2, represents the pulse obtained from the resistance 35 in the cathode circuit of valve ll of multivibrator IS. The instantaneous amplitude of the intelligence wave of channel i is assumed to raise the pulse Tl curve I, so that the multivibrator is triggered when the trapezoidal pulse passes through the voltage value represented by point 111. The multivibrator triggers 8 and produces the leading edge of pulse 4H whose duration depends upon the constants of the multivibrator. At the next cycle of the distributor the intelligence wave amplitude raises or lowers the trapezoidal pulse, for example, so as to trigger the multivibrator at the point 112 (curve Dto produce the pulse 2H curve h. The pulses IH and 2H are assumed tobe of the same duration, but it will be observed that the leading edges are moved with respect to the mean pulse positions represented by the arrows CHI.

Curve It shows the pairs of positive and negative pulses IKP, IKN, 2K? and 2KN obtained by differentiating the pulses IHI and 2H! respectively by the capacity 26 and resistance 31, Fig. 1. Curve kp shows the train of positive pulses IKP, 2KP'after eliminating the negative pulses by means of diode 39. Since IX? and 2K? are produced from the leading edges of pulses .IHI and 2Hl which are time phase modulated, the train of pulses IKP, 2KP are also time phase modulated. v

The method of modulating just described is well known but usually a sawtooth wave form is used in place of the trapezoidal waveform. The trapezoidal waveform, or other waveform having waveform with perpendicular leading and trailling edges, has the advantage that the channel pulses are modulated in time within the period determined by the leading and trailing edges and cannot go beyond them providing the amplitude of these edges is great enough.

Referring back to curve'g, Fig. 2, it will be seen that if the amplitude of the intelligence wave is sufficiently negative (utilising a positive trapezoidal pulse, or sufliciently positive utilising a negative trapezoidal pulse), the trapezoidal pulse will not attain the triggering voltage of the multivibrator and the multivibrator will thus not operate during the current channel period and no pulse will be transmitted in that channel. This may cause some inconvenience and have perturbing effects on the channel multivibrator and on the whole multi-channel system. In the case of the multivibrator, when it is not excited periodically, for'example due to the fact that pulse is missing from the train of exciting pulses, charges accumulate irregularly on the condensers and produce varying bias voltages which affects the operation of the multivibrator for the succeeding channel periods, giving incorrect time modulation of the pulses. In,order to overcome such disadvantages a short duration pulse of suitable amplitude is therefore added at the end of the trapezoidal pulse as shown in curve m Figure 2. Such a pulse is produced by coil-'29 as hereinbefore explained with reference to Figure 1. From curve m it will be observed that if the trapezoidal pulse falls below the triggering voltage represented by the line TVI, the short duration pulse Mi will cause the multivibrator to trigger and produce a pulse at the end of the channel period.

While in the foregoing description combined waves have been referred to, it will be understood that the trapezoidal or other form of combined 1 wave, may not in fact exist, and the same eiiect may be obtained for example by applying the rectangular pulse and. the auxiliary pulse waveform (sawtooth etc.) individually to an amplifier in such manner that the effects due to the waves in the output of the amplifier are additive.

At the receiver, the time-phase modulated pulses are converted by means of known buildback circuits into duration modulated pulses which are passed through a low pass filter to produce the intelligence wave. This method of demodulation introduces no amplitude distortion provided the cut-off frequency is lower than fe/2, where f. is the channel pulse repetition frequency or the distributor cyclic frequency. A very small amplitude distortion appears at submultiple frequencies of jg, but these distortions are usually very small of the order of 2.0% for Iii/3, .0496 for fe/4 and completely negligible for the other subharmonics.

Furthermore, provided the product of the highest component frequency of the intelligence wave to be transmitted into the maximum timephase modulation is small, the overall harmonic distortion is also small. Figs. 3, 4 and 5 give numerical values for the harmonic distortion produced respectively by the modulation process employing a trapezoidal waveform as hereinbefore described, (Fig. 3), the demodulation process, (Fig. 4) and the overall distortion produced by the modulation process followed by the demodulation process (Fig. 5). In these figures the ordinates represent percentage harmonic present in the modulation or demodulation processes and in Figs. 3 and 4 the abscissae A are X product of time modulation AT into angular frequency F==2r x frequency. The abscissae B represent the time modulation AT in micro-seconds for F=21r1,5 kc. (kilocycles). In Fig. the abscissae represent X0 for full modulation. It will be observed that the modulation and the demodulation processes taken individually produce an appreciable and similar amount of second harmonic. Fortunately the second harmonic components introduced at the two stages are of opposite signs, so that the second harmonic distortion produced at modulation tends to neutralise the distortions produced at demodulation. This fact explains why the second harmonic present in a wave after the modulation and demodulation processes has a relatively small amplitude, especially for small values of X0. The third harmonic component introduced at the stages has the same sign at the modulation stage as at thedemodulation stage so that the third harmonic amplitude after demodulation is greater than the amplitude of third harmonic component introduced at either stage. this increase in distortion for small values of X0.

It may be observed from Fig. 5 that the harmonic distortion increases with X. Thus, the systems must be designed to keep X0 as small as possible. However, for a given signal frequency bandwidth, the only way to reduce X0 is to reduce the depth of time modulation. This is possible as long as the depth of modulation is great compared with the depth of the fluctuating modulation produced by noise and instability in the circuits. Reducing the depth of modulation reduces the signal/noise ratio so that there is a practical limit. It has been found that the signal/noise ratio is still greater than 55 db. for a depth of modulation of i1 microsecond. Figure 5 shows that this value produces a completely negligible harmonic distortion.

To arrive at the curves of Fig. 3 it has been assumed that the rectangular waveform channel sector pulses or gating pulses used to produce the trapezoidal waveform and the trapezoidal waveform have a perfect shape. In other words, it has been assumed that the rising effective portion of the trapezoidal pulse wave is a straight is of no importance But these amplitudes are so small that 1 line and that the top of the selector or gating pulse wave form is a straight line perfectly parallel to the time axis.

If this is not the case, harmonic distortion may occur. In practice the pulses are never perfect because they are built by means of valves and networks. If the circuit is suitably designed the distortion produced by the valves can be reduced to a very small percentage since in pulse technique the valves work mainly as relays below cut-oil or at full saturation, but the networks have time constants and the pulses are distorted by passing through them. In many cases the design of distortionless circuits presents some important disadvantages and in others the distortion is considerable and unavoidable.

The necessity of producing perfectly shaped trapezoidal or rectangular wave forms is avoided according 'to a feature of the invention by employing pulses at modulation and demodulation whose amplitudes vary exponentially with time during effective time periods of the pulses and arranging that the exponential variations are substantially equal at the modulator and demodulator.

This feature of the invention is not limited to modulators using trapezoidal waveform and demodulators using gating pulses of rectangular waveform but may be applied in all cases where a time modulation is produced under the control of an arrangement involving the time constant of a resistance capacity or resistance inductance combination and to demodulators controlled by an arrangement involving the time constant of a resistance-capacity or resistance inductance combination.

In Fig. 6 a modified form of trapezoid pulse generator is shown, and may be arranged to feed pulses of trapezoidal waveform to a modulator as hereinbefore described in relation to Fig. 1. The arrangement in Fig. 6 only differs from the arrangement 22, 23, 24 shown in Fig. 1 in that a further resistance 43 completes a chain of resistances with 23' and 24' across the H. T. supply to enable a bias to be obtained on the cathode.

Pulses as indicated at M are obtained from a point on the distributor network to bring the channel cyclically into use. These pulses are herein called selector pulses and as explained in my co-pending United States application, Serial No. 602,803, filed July 2, 1945, for Multi-cl'iannel electrical pulse communication systems, now Patent No. 2,462,111, issued Feb. 22, 1949, are more or less distorted and consequently produce very distorted trapezoid pulses in the output circuit of valve cautions are taken. If the amplitude of the selector pulses 44 is very great grid current will pass in valve 20' and the anode current in valve 20' will remain nearly constant until the selector pulse disappears.

Assuming this anode current is constant, the current will flow in resistance 23' and condenser 22'. Resistance 23' will produce the rectangular part of the trapezoid waveform and condenser 22' the triangular part, but condenser 22' must beshunted by resistance 24 for the discharge period and unless 24 has a very great value, the shape of the triangular part will be distorted and the linear portion of the triangular waveform will be replaced by an exponential curve waveform.

Referring to Fig. 8, the voltage V across condenser 22' of capacity C is proportional to the 20' (Fig. 6) unless some pre-' 11 current i flowing in resistance 24' of value R and this current is given by the differential equation:

di i I afi'e c 1) where I is the valve anode current.

The corresponding operational equation is:

i=I/(RC.p+1) (2) where p is the operational symbol, which gives the solution:

t V=R,-=V [1exp. (a) with Vo=RI. This is an exponential curve as shown at 46 on Figure 8!).

Let to be half the duration of the trapezoid pulse. Shifting the origin of time at t=to and putting T=t-to, gives:

If T is small compared to RC, Formula 5 can be written to a first approximation:

Formula 6shows that the second harmonic distortion at modulation is smaller than if T is smaller than 1% of the time constant RC. Assume a modulation depth equal to 2.5 microseconds, this means that RC must be greater than 250 microseconds. For a repetition frequency of kc./s, this means that RC must be much greater than the repetition period.

This can easily be done, but it has in practice some disadvantages. When a trapezoid pulse appears, condenser 22', Fig. 8a is not completely discharged. In fact the voltage drop is equal to the voltage rise during the trapezoid-build up. Now if the time constant RC is, say, 300 microseconds, the remaining charge of the condenser when a new trapezoid pulse occurs is equal to about twice the voltage drop. If it is desired to limit harmonic distortion to lower than /2/ RC will have to be increased still more and the remaining voltage across 22' will be still greater. This voltage constitutes a constant bias voltage on the cathode of the trapezoid generator valve 20, Fig. 1, and determines at what voltage value in the amplitude of the selector pulse, the valve will begin to pass current. In other words said bias determines the lower limit of the slice cut, so to speak, by the valve 26 on the applied selector pulses as indicated by the broken lines 41 and 48, Fig. '7, 48 representing the saturation voltage of valve 20. Now the value of this lower limitin line 41 must have a value which is a function of the shape and amplitude of the selector pulses as is explained in the aforementioned United States application Ser. No. 602,803. In practice the bias voltage of valve 20 is of the same order of value as the increase of voltage during the build-up of the trapezoid pulses. It is clear that since the bias voltage has to obey two very different requirements, only a compromise is possible.

Other disadvantages appear also when trying to obtain a perfectly shaped trapozoid pulse. In

this case, for instance, the variable duration pulse wave form obtained at demodulation must have the portion between the leading and trailing edges substantially linear and parallel to the time axis. This condition however is not fulfilled in most types of demodulator circuits, so choice of demodulator circuits is limited.

Another important disadvantage in the trapezoidal waveform generator when used coupled as the input to a multivibrator circuit as shown in Fig. 1 is due to the grid current which appears in the multivibrator circuit when the latter triggers over and this grid current discharges the condenser 22 to some extent. Assume that value 20 of Fig. 1 produces an anode current, of say, 30 milliamps during the trapezoid pulses; ii the grid current appearing at the modulator multivibrator is of the order of 3 milliamps and if the modulator triggers at the middle amplitude voltage of the trapezoid pulse, the discharge due to the grid .current will result in a reduction of about 5% of the build up voltage and a great portion of this voltage will affect the height of the next trapezoid pulse thus shifting the position in time or time phase of the corresponding channel pulse in a multi-channel system as well as producing a harmonic distortion.

This disadvantage becomes more important in the case of the improved trapezoid waveform generator and modulator circuit represented in Fig. 9.

Referring again to the trapezoid generator of Fig. 6, the voltage swing of the cathode is equal to the voltage amplitude of the trapezoid pulses and may be very great, for instance, of the order of volts.

The amplitude of the efiective portion between lines 41-48 cut on the selector pulses (Fig. 'l) is consequently not very small and the shape of the resulting trapezoid pulses is not very good. In the improved circuit of Fig. 9 the cathode 49 of the input valve 50 is at a fixed bias potential and the trapezoid waveworm is generated by the constant anode current of the pentode valve 50 flowing through capacity Si in the circuit resistancecapacity 5253, resistance 54 corresponding to 22', 23' and 24' in Figure 6. Resistance 55 is used only to supply the H. T. voltage to the anode of valve 50. The trapezoid pulses obtained in 52, 53, 54 are negative and are applied-to the cathode 56 of the first valve 51 shown as a pentode of the modulator multivibrator 58, and the modulating signal is applied for simplicity on one of the grids of valve 51 as indicated diagrammatically at 59 by a microphone in series with the feedback from the output valve 60. An amplifying valve BI is inserted between the input and output valves 51 and 60 of the multivibrator circuit.

When the modulator multivibrator triggers the current flowing in condenser 53 has a considerable amplitude and the disadvantage referred to hereinbefore on account of the grid current in valve 51 is so great that a considerable harmonic distortion appears in the output. To avoid this distortion the time constant represented by the product of resistance R of 54 and capacity Co! 53 must be small compared with the pulse repetition period so that condenser 53 will discharge completely before the reset pulse occurs. Under these conditions, however, as explained hereinbefore, the trapezoid pulse has a very bad shape.

This distortion introduced into the signal wave obtained at the receiver on account of this bad shape of trapezoidal waveform is however completely eliminated by suitably designing the receiver demodulators so that variable duration pulses obtained in the demodulation process have the shape of an exponential curve between the .leading and trailing edges of the pulses, said exponential curve having the same exponent as the exponential part of the trapezoidal portion of the modulator at the transmitter.

Before explaining the reason for this, it is ecna eiaeel R66+R67+R68 C63 in which R and C represent the values of resistances and capacities indicated by the suflixes.

If the trapezoid and'demodulator pulses are of perfect shape, when a signal of amplitude V volts is applied at the transmitter, the channel pulse will be shifted from its normal position by T microseconds when T is proportional to V. At

the receiver the demodulator pulse will increase its width by T microseconds, and the amount of current supplied by the pulse to the low-pass filter will be equal to a constant plus an amount proportional to T, that is to say, to V.

The same result can be obtained when utilising exponentially shaped pulses if the exponential time constants are substantially equal at the transmitter and receiver.

If the trapezoid pulse has an exponential build up. when a signal voltage V is applied, the time shift T is given by Equation 4 above:

Now a suitable amplitude A=f( T must be determined for the duration modulated pulses produced in the demodulator. This amplitude should be equal to a constant plus a quantity proportional to V.

This means that A must be such that:

J A.dT=M+NV 7 where M and N are constants.

Replacing V by the value given by (4) gives f A.dt=1vI+NV [1- 0: .exp. (-T/RC) 1 (s This equation is satisfied if:

This means that the part of the demodulator pulses between the leading and trailing edges must drop slowly according to an exponential law with a time constant equal to RC.

It should be observed that this demonstration is correct only to a first approximation, since the integration should be replaced by a Fourier integral to obtain a very accurate result. The calculation then becomes very complicated. For this reason it is more practical to take the first approximation and make a final adjustment of desirable to emphasise that even for a very comthe distortion to the greatest possible degree.

For the same reason, it is advisable where possible not to make the time constant RC too small. For instance with a channel period. of 100 microseconds and a time modulation depth of 2.5 microseconds, a value of RC of the order of microseconds will give good results.

In the above demonstration it has been assumed that the current I feeding the network is constant. Referring again to Figs. 6 and 9 if the selector pulse is badly shaped this constant current I will not be obtained even if resistances ll, 69 respectively in the grid feed circuits are great. An improvement can be obtained by limiting voltage applied to the grid by means of a diode 10 or other one way device connected as shown in Figure 9. The cathode of the diode 10 must be suitably biassed in order to get the best results, but for simplicity, in Fig. 9 the diode has been shown as connected directly to the cathode of valve 50.

Another reason why the current I supplied by valve varies is that the plate voltage progressively drops during the presence of the trapezoid pulse. The plate current variation is, however, small andof the order of less than of I. This variation may be compensated for by varying the time constant RC at the receiver slightly from the theoretical value, to balance as nearly as possible other variations at the transmitter and receiver. resulting from the plate current variation at the transmitter.

Demodulator circuits embodying the invention will now be described.

Fig. 11 shows a gating pulse generator valve H connected to a demodulator multivibrator comprising valves 12, 13, I4. This circuit is very similar to the modulator shown in Fig. 9. Valve 1|, Fig. 11 corresponds to valve 50, Fig. 9, and feeds the capacity resistance network 15, 16, 11 in the cathode circuit of valve 72 to produce the exponential waveform. The valve H is fed with pulses of rectangular waveform, as indicated at 18, for example, from a distributor delay network as herein fore mentioned. Fig. 12a shows the circuit equivalent of network l5, 116, I1 and bearing in mind that the pulse produced in the anode circuit resistance and applied to the circuit 15, 16, I1 is negative the pulse produced across resistance 75 will be as shown in Fig. 12b. Such a pulse is applied to the cathode circuit of the multibrator valve 12 which triggers the voltage represented by point 19 and the pulse is terminated at that point. In the circuit shown, the channel pulses are applied on a grid of valve 14 numeral all denotes the leading edge of the pulse or commencement of the ,gate" period, 3| denotes the trailing edge and 82 denotes the termination of the gate period. This point 19 can occur at any time between the time limits represented by and 82. It will be clear that the shape of the waveform of the gating pulse between the limits so and 82 is an exponential curve with a time constant equal to Cl? (Rl5'+R76'). If this time constant is equal to the time constant at the transmitter, for example C53 (RM-PR5!) Figs!) the distortion will be minimum.

Figure 13 shows a type of demodulator utilising gas-filled valves producing the sametype of pulses as shown in Figure 12?).

This demodulator comprises two gas discharge valves 83 and 8G. The capacity 85 charges up from the H. T. supply during the interval between pulses. Selector pulses are applied to the grid of valve 83 as indicated at 86 and cause this valve to ignite and provide a discharge path for only be ignited when 83 is operated. The channel pulses are applied to the control grid of valve 8| as indicated at 88 and the first to occur after valve 88 is made conductive by a selector pulse causes valve 84 to ignite and short circuit the resistance 81. This causes the anode voltage of 88 to drop due to increased current and 83 is extinguished terminating the pulse for example at a point such as 19 (Fig. 12b). The pulse obtained at the terminal 88 is thus represented by the shaded portion of Figure 12b. It will be observed that only that channel pulse immediately following a selector pulse can operate valve 84 and hence the channel pulses are directed to their proper channels automatically since the trains of selector pulses of the respective channels coming from a distributor are time phased with respect to each other. I

In practice the value of resistance 81 and capacity 85 can be adjusted at the receiver to eliminate the greatest amount of distortion.

What is claimed is:

1. A multi-channel electrical communication system comprising a distributor arrangement including a delay network, means for feeding to the input terminals of said network pulses of rectangular waveform, and means for obtaining chanel selector pulses of trapezoidal waveform of duration equal to a channel period from successive points along said network, and means for applying a trapezoidal pulse and the intelligence wave of a channel to a pulse generator to produce time modulated pulses in accordance with the varying amplitude of said intelligence wave.

2. A multi-channel electrical communication I system in which the intelligence wave of each channel is transmitted as a time modulation of a series of electrical pulses comprising a distributor arrangement including means for delivering channel selector pulses of rectangular waveform, means for deriving from said selector pulses, pulses of trapezoidal waveform, a pulse generator of the electron discharge type, and means for 241- plying said trapezoidal waveform and the intelligence wave of the channel to said/pulse generator to produce time modulated pulses.

3. A multi-channel system as claimed in claim 2, said means for producing a trapezoidal waveform comprising a resistance-capacity network in which said capacity is shunted by a resistance, means for feeding said rectangular selector pulse to said network and means for taking said trapezoidal waveform from across said capacity in series with a resistance.

4. A multi-channel system as claimed in preceding claim 2 comprising means for superimposarrangement for rendering said channels operative cyclically and successively having a delay device fed at the input terminals with pulses of rectangular waveform, and means for each channel for producing from pulses of rectangular waveform pulses of'trapezoidal waveform means for applying pulses obtained at successive points along said device to respective ones of said trapezoidal waveform generators, a pulse generator for each channel and means for applying the trapezoidal waveforms and the intelligence waves of the channels to respective pulse generators to produce time modulated pulses.

6. A system as claimed in claim 5, said generator of pulses of trapezoidal waveform comprising a resistance-capacity network.

7. A multi-channel system as claimed in claim 5 further comprising means for applying at the end of each channel period a safeguard pulse of sufficient amplitude to operate said pulse generator in the event of reduction by the intelligence waves of the amplitude of the combined rectangular and other waveform or trapezoidal waveform, below the effective operating value.

8. A multi-channel electrical communication system in which the intelligence wave of each channel is transmitted as a time modulation-of a series of electrical pulses comprising a distributor arrangement providing time phased pulses of rectangular waveform for rendering said channels cyclically and successively operative, a pulse generator of the mutlivibrator circuit type for each channel, an electron discharge valve amplifier for ing substantially at the end of the trapezoidal I pulses a short sharp pulse of sumcient amplitude to affect the modulator to safeguard against possible neutralising of said trapezoidal pulses by the channel intelligence wave.

5. A multi-channel electrical communication system in which the intelligence wave of each channel is transmitted as a time modulation of a series of electrical pulses comprising a distributor each channel, a resistance-capacity network for the production of trapezoidal waveforms in the cathode circuit of one of the valves of said multivibrator circuit, means for feeding said amplifier with said rectangular pulses from said distributor arrangement, means for feeding the rectangular pulses from the anode-cathode circuit of said amplifier to said network and means for applying the intelligence waves of the channels to respective multivibrator circuits to control the instants of triggering of said multivibrator circuits in accordance with the instantaneous amplitudes of said intelligence waves to produce time modulated pulses.

9. A multi-channel electrical communication system as claimed in claim 5 further comprising means for eliminating effects due to distortion of the trapezoidal waveform, having an exponential variation of voltage or current with time.

10. A mvulti-channel electrical communication system as claimed in claim 5 further comprising means for eliminating effects due to distortion of the trapezoidal waveform having an exponential variation of voltage or current with time, including means at the receiver for producinc duration modulated pulses under control of the received time modulated pulses, said duration modulated pulses having an exponential variation of amplitude with time whose exponent is equal to the exponent of the distorted trapezoidal waveform at the transmitter.

11. A multi-channel electrical communication system as claimed in claim 5 further comprising means for eliminating effects due to distortion of the trapezoidal waveform having an exponential variation of voltage of current with time, said means at the receiver for producing duration modulated pulses including a capacity-resistance network having the same time constant as the trapezoidal waveform generator network at the transmitter and means for applying the potential across a series connected resistance-cavoltage with aesasos l7 pacity of said network as a controlling potential to trigger a pulse generator.

12. A multi-channel electrical communication system as claimed in claim further comprising means for eliminating eifects due to distortion or the trapezoidal waveform having an exponential variation of voltage or current with time, said means at the receiver for producing duration modulated pulses including a capacity-resistance network having the same time constant as the trapezoidal waveform generator network at the transmitter, means for applying the potential across a series connected resistance-capacity of said network as a controlling potential to trigger a pulse generator, a demodulator comprising a multivibrator type of circuit having one stable condition and having said series resistance-capacity of said network in the cathode grid circuit of one valve, means for feeding said network with pulses from a distributor system to trigger the circuit into its unstable condition, means for applying the received train of transmitted distorted pulses to trigger the circuit hack into its stable condition, and means for deriving from the duration modulated pulses obtained at any electrode of the multivibrator, the intelligence wave.

13. A multi-channel electrical communication system as claimed in claim 5 further comprising means for eliminating eflects due vto distortion of the trapezoidal waveform having an exponential variation of voltage or current with time, said means at the receiver for producing duration modulated pulses including a capacity-resistance network having the same time constant as the trapezoidal waveform generator network 'at the transmitter, means for applying the potential across a series connected resistance-capacity of said network as a controlling potential to trigger a pulse generator, a first gas discharge valve having in its cathode circuit a series connected resistance-capacity combination having a time constant equal to the time constant of the trapezoidal waveform generator at the transmitter,

Pulses from a disarrangement and render it conduct a second gas discharge valve connected across said resistance and means for applying to a control electrode of said second said resistance to cause both valves to ex nguish. and means for deriving from the duration modulated pulses obtained from the rminals or said resistance, the intelligence waves.

rangement for producing in accordance with the instan of the intelligence wave inclu network having a time constant in the modulator cult introducing an exponential variation of time and consequent distorting comcontrol grid of "said to trigger said first valvev e 18 ponents into the transmitted signal characterised in this that at the receiver the demodulator comprises means for producing under the control pulses at the receiver includes a resistance-capacity network.

16. An electrical communication system as claimed in claim 14 wherein said modulator includes a resistance-capacity network means for feeding said network with pulses of rectangular waveform to produce across said capacity a trapezoidal waveform, and means for applying said trapezoid-a1 waveform together with the intelligence wave to control a pulse generator for the production of time modulated pulses.

17. A system as claimed in claim 14 having a demodulator comprising a multivl-brator circuit arrangement having said capacity-resistance network connected in the cathode grid circuit oi one valve, means for feeding said network with pulses of rectangular waveform at the operating frequency to trigger the multivibrator over, means for applying the received train of transmitted pulses to trigger the circuit back to its original condition and means for deriving from the duration modulated pulses in the output of said multivibrator, the intelligence wav 18. A system as claimed in claim 14 having a demodulator comprising a first gas discharge valve having a series connected resistance capacity combination in its cathode circuit, means for applying to the control grid of said first valve a series of pulses having the operating repetition frequency of the received time modulated pulses,

, 4 to trigger said first valve and render it conducting, a second gas discharge valve connected across said resistance and means for applying to a control electrode of said second valve the received train of time modulated pulses to i nite said second valve when the first valve is passing current, theigni-tion of said second valve placing a short circuit across said resistance valves to extinguish and means for terminals of said resistance, the intelligence wave.

MAURICE -MOISE LEVY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

